Nitrenes singlet dynamics

Once the spectroscopy and dynamics of parent singlet phenylnitrene were understood, we began a systematic study of the effect of substitution on the kinetics of singlet phenylnitrenes. For most of the aryl azides of interest " the rate constants of singlet nitrene decay and product formation (triplet nitrene and/or ketenimine) are the same (Fig. 9). With these nitrenes, cyclization to substituted benzazirines is the rate-limiting step of the process of nitrene isomerization to ketenimine in a manner similar to the parent phenylnitrene. The only exception, o-fluorophenylnitrene, will be examined in detail in the last section of this review. 

One of the most interesting products obtained from the capture of singlet nitrene 16e is ylide 22e produced by photolysis of 15e in pyridine (Scheme 4). Ylide 22e has a very intense absorption band with maximum absorption at 390 nm. The pyridine-ylide method was successfully used by our group to probe the dynamics of the fluoro-substituted singlet arylnitrenes - " -" ... 

Therefore, as in the case of parent phenyl azide 47 and its simple derivatives, the photochemistry of polynuclear aromatic azide, especially that of naphthyl azides 79 and 80, is now well understood. Specifically, the dynamics of the primary photophysical processes as well as the subsequent photochemical steps have been directly investigated using a variety of modem and conventional experimental techniques and compntational chemistry. It is clear now, that the difference between the photochemistry of phenyl azide (and its simple derivative) and polynuclear aromatic azide is caused mainly by the difference in the thermodynamics of the singlet nitrene rearrangement to azinine type species. 

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